In a man machine control system the task of the human operator is continually to decide upon and make control adjustments on the basis of his sensory perceptions to insure that the output of the system remains appropriate to the inputs. In applications where it is not feasible to entirely replace the man by a automatic controller, a common design goal is to ensure that the transfer function required of the man is mathematically always as simple as possible. For steering control of ships it is not feasible to eliminate the human operator from the continuous control loop under all circumstances. Displays presently and previously in use for ship steering control have not met the design goal of simplifying the transfer function required of the human operator. Part of the reason has been the lack of a suitable display system.
The helmsman of a ship typically has his choice of two modes of steering control: hand electric or autopilot. When the autopilot mode is selected, the ship is steered automatically by the autopilot which determines the error between ordered and actual heading and outputs an appropriate signal corresponding to an ordered rudder position. The task of the helmsman is to adjust the ordered heading as necessary and to monitor the operation of the autopilot to determine if it is working properly. Displays typically available consist of a compass, a helm angle (ordered rudder angle) indicator and an actual rudder angle indicator. The autopilot solves the differential equations necessary to determine the appropriate helm angle. In the most conventional autopilot systems the helmsman cannot modify the helm angle ordered by the autopilot without turning the autopilot off and switching to the hand electric mode of steering control, thereby depriving himself of the computational capabilities of the autopilot. When the hand electric mode is selected, the displays available to the helmsman are signals from a compass, an actual rudder angle indicator and an indicator of the helm angle. The helm angle is continuously adjusted by the helmsman without benefit of the computational capabilities of the autopilot. Although determining the reasonableness of the helm angle is a task suitable for a man, the prior art display systems require him to keep the autopilot off during critical manuvers to prevent an unreasonable rudder angle which may be dangerous. For example, when two ships are moving close to each other, a temporay malfunction of an autopilot could cause a rudder angle which would result in a collision. Such a malfunction possibly would not be discovered in time to avoid a collision.
The basic design goal is to make the helmsman's task as simple as possible.
To maintain a given heading, a ship control system requires that the optimum helm wheel angle be frequently recalculated and displayed to the operator. Such helm angle calculations will take into account the ship speed, the difference between the actual heading and the ship's desired heading, and the maneuvering characteristics of the ship.
It is desirable to present the helmsman in some usable fashion with a calculated value of optimum helm angle. Two types of displays can be used for this purpose. A pursuit type display will display the calculated optimum value of the quantity being controlled as well as the actual value. A compensatory display is a three position discrete indicator which shows that the quantity being controlled should be changed in one direction, changed in the opposite direction or not changed at all. One example of such a display would be three lights with one light indicating a left turn, another light a right turn and the third light indicating no turn is needed. Neither of these two types of displays will be completely satisfactory by itself. The compensatory display does not give any indication to the operator of the size of change that is required. The pursuit type display is generally not good for indicating both when very large changes in the controlled quantity are needed and when very small changes are needed. A display is needed that will both indicate the size of change that is needed in the controlled quantity and also effectively indicate the required direction of that change regardless of its size.